The field of the invention is nuclear magnetic resonance imaging methods and systems. More particularly, the invention relates to the use of fast spin echo NMR echo pulse sequences to produce images of moving spins.
Any nucleus which possesses a magnetic moment attempts to align itself with the direction of the magnetic field in which it is located. In doing so, however, the nucleus precesses around this direction at a characteristic angular frequency (Larmor frequency) which is dependent on the strength of the magnetic field and on the properties of the specific nuclear species (the magnetogyric constant .gamma. of the nucleus). Nuclei which exhibit this phenomena are referred to herein as "spins".
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B.sub.0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. A net magnetic moment M.sub.z is produced in the direction of the polarizing field, but the randomly oriented magnetic components in the perpendicular, or transverse, plane (x-y plane) cancel one another. If, however, the substance, or tissue, is subjected to a magnetic field (excitation field B.sub.1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, M.sub.z may be rotated, or "tipped", into the x-y plane to produce a net transverse magnetic moment M.sub.t, which is rotating, or spinning, in the x-y plane at the Larmor frequency. The practical value of this phenomenon resides in the signal which is emitted by the excited spins after the excitation signal B.sub.1 is terminated. There are a wide variety of measurement sequences in which this nuclear magnetic resonance ("NMR") phenomena is exploited.
When utilizing NMR to produce images, a technique is employed to obtain NMR signals from specific locations in the subject. Typically, the region which is to be imaged (region of interest) is scanned by a sequence of NMR measurement cycles which vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques. To perform such a scan, it is, of course, necessary to elicit NMR signals from specific locations in the subject. This is accomplished by employing magnetic fields (G.sub.x, G.sub.y, and G.sub.z) which have the same direction as the polarizing field B.sub.0, but which have a gradient along the respective x, y and z axes. By controlling the strength of these gradients during each NMR cycle, the spatial distribution of spin excitation can be controlled and the location of the resulting NMR signals can be identified.
The concept of acquiring NMR image data in a short time period has been known since 1977 when the echo-planar pulse sequence was proposed by Peter Mansfield (J. Phys. C.10: L55-L58, 1977). In contrast to standard pulse sequences, the echo-planar pulse sequence produces a set of NMR signals for each RF excitation pulse. These NMR signals can be separately phase encoded so that an entire scan of 64 views can be acquired in a single pulse sequence of 20 to 100 milliseconds in duration.
A variant of the echo planar imaging method is the Rapid Acquisition Relaxation Enhanced (RARE) sequence which is described by J. Hennig et al in an article in Magnetic Resonance in Medicine 3,823-833 (1986) entitled "RARE Imaging: A Fast Imaging Method for Clinical MR." The essential difference between the RARE sequence and the EPI sequence lies in the manner in which echo signals are produced. The RARE sequence utilizes RF refocused echoes generated from a Carr-Purcell-Meiboom-Gill sequence, while EPI methods employ gradient recalled echoes.
Both of these "fast spin echo" imaging methods involve the acquisition of multiple spin echo signals from a single excitation pulse in which each acquired echo signal is separately phase encoded. Each pulse sequence, or "shot," therefore results in the acquisition of a plurality of views, and single shot scans are commonly employed with the EPI method. However, a plurality of shots are typically employed to acquire a complete set of image data when the RARE fast spin echo sequence is employed. For example, a RARE pulse sequence might acquire 8 or 16 separate echo signals, per shot, and an image requiring 256 views would, therefore, require 32 or 16 shots respectively.
There are a number of well known NMR techniques for measuring the motion, or flow of spins within the region of interest. These include the "time-of-flight" method in which a bolus of spins is excited as it flows past a specific upstream location and the state of the resulting transverse magnetization is examined at a downstream location to determine the velocity of the bolus. This method has been used for many years to measure flow in pipes, and in more recent years it has been used to measure blood flow in human limbs. Examples of this method are disclosed in U.S. Pat. Nos. 3,559,044; 3,191,119; 3,419,793 and 4,777,957.
A second flow measurement technique is the inflow/outflow method in which the spins in a single, localized volume or slice are excited and the change in the resulting transverse magnetization is examined a short time later to measure the effects of excited spins that have flowed out of the volume or slice, and the effects of differently excited spins that have flowed into the volume or slice. Examples of this method are described in U.S. Pat. Nos. 4,574,239; 4,532,474 and 4,516,582.
A third technique for measuring motion flow relies upon the fact that an NMR signal produced by spins flowing through a magnetic field gradient experiences a phase shift which is proportional to velocity. This is referred to in the art as the "phase modulation" technique. For flow that has a roughly constant velocity during the measurement cycle the change in phase of the NMR signal is given as follows: EQU .DELTA..phi.=.gamma.M.sub.1 v
where M.sub.1 is the first moment of the magnetic field gradient, .gamma. is the gyromagnetic ratio and v is the velocity of the spins. To eliminate errors in this measurement due to phase shifts caused by other sources, it is common practice to perform the measurement at least twice with different magnetic field gradient moments as described in U.S. Pat. No. 4,609,872. The difference in phase at any location between the two measurements is then as follows: EQU .DELTA..phi.=.gamma..DELTA.M.sub.1 v
By performing two complete scans with different magnetic field gradient first moments and subtracting the measured phases in the reconstructed image at each location in the acquired data arrays, a phase map is produced which accurately measures the velocity of constantly moving spins. Such phase maps may be processed to reconstruct an angiogram in which stationary tissues are dark and the vascular tree is bright.
The phase modulation method for producing an NMR angiogram typically employs a conventional gradient recalled echo pulse sequence and requires up to a few minutes to complete a scan. Compared to spin echo pulse sequences, gradient recalled echo sequences may suffer from reduced SNR and reduced immunity to off-resonance effects such as field inhomogeneity and magnetic susceptibility. When the phase modulation method is applied to conventional spin echo pulse sequences, as to overcome one or more limitations of the gradient recalled echo method, many minutes are required to complete the scan. The application of the phase modulation technique to a fast spin echo pulse sequence, on the other hand, would allow completion of a scan and reconstruction of an angiogram in less than a minute.